1. Field of the Invention
The present invention relates to an image printing apparatus and particularly to an image printing system and an image processing method which perform dot printing for pseudo halftone representation by means of multi-pass printing.
2. Description of the Related Art
Recently, an effort to realize a higher resolution and a smaller droplet has been promoted in an ink jet printing apparatus which is capable of representing desired density on a printing medium by means of pseudo halftone reproduction, and there is devised a measure for carrying out the sequence of image processing for this purpose as simply and speedily as possible. For example, multi-value density data of an original image (256 gradation levels) for a pixel which has a relatively low resolution is quantized into lower level multi-value density data (17 gradation levels), and then converted into binary density data suitable for a resolution which can be printed by a printing apparatus, after having been subjected to various kinds of processing.
As a method of converting the low resolution multi-value (17 gradation levels) data into the binary data, there is known a data processing method called index processing. The index processing selects one of plural dot arrangement patterns which are preliminarily prepared as shown in FIG. 4 according to the value of the multi-value data, and thereby converts one multi-value data into plural binary data. The result of such index processing determines a pixel (1) which actually prints a dot and a pixel (0) which does not print a dot, and desired density can be represented on a printing medium by means of a pseudo halftone.
An actual dot is printed by the main scan of a printing head on a pixel of the printing medium where dot printing has been determined by the index processing, and at this time, the ink jet printing apparatus frequently employs a multi-pass printing method for improving image uniformity. The multi-pass printing method is a method of printing the plural dots which can be printed in one main scan of the printing head, by dividing them into plural main scans. At this time, a mask pattern, for example as shown in FIG. 5, is used which preliminarily defines the pixel (1) that allows printing and the pixel (0) that does not allow printing in one main scan of the printing head. Then, by a logical product operation between the allowance (1)/non-allowance (0) defined by such a mask pattern for the printing of each pixel and the printing (1)/non-printing (0) determined by the index processing, the data to be actually printed by the printing head is determined for each main scan.
In a printing system using both of the index processing and the mask processing, there has been proposed a method of carrying out various kinds of printing control by causing these two kinds of processing to be associated with each other. For example, Japanese Patent Laid-Open No. 2007-168202 discloses a configuration to cause a satellite not to be conspicuous in a high speed printing mode by making the dot arrange pattern different according to a set printing mode (mask pattern). Further, Japanese Patent Laid-Open No. 2008-173969 discloses a method of preparing the dot arrangement pattern and the mask pattern in association with each other and thereby controlling the order of ink application for the same image area in an ink jet printing apparatus which performs printing by using plural kinds of ink.
Meanwhile, for obtaining the advantages of such Japanese Patent Laid-Open No. 2007-168202 and Japanese Patent Laid-Open No. 2008-173969, it is necessary to promise (fix) a positional relationship between the dot arrangement pattern and the mask pattern which are prepared in association with each other. That is, in Japanese Patent Laid-Open No. 2008-173969, for example, when a shift occurs in the positional relationship between the dot arrangement pattern and the mask pattern, it becomes impossible to control the order of ink application for a unit pixel which is formed by one dot arrangement pattern.
Recently, however, each size of the dot arrangement pattern and the mask pattern has been expanded both in the main scan direction and the sub-scan direction and also the content thereof has become complicated. Accordingly, raster image processing generating raster data while performing the index processing and the mask processing to be performed for the multi-pass printing are frequently configured as independent jobs, respectively, and there occurs even a situation in which the positional relationship is not always fixed between the both kinds of processing.
Such a situation will be explained below in detail. For example, the raster image processing performs a job of generating binary data by the index processing and compressing this binary data. Further, the raster image processing searches image data generated by an application, and generates a command for moving the printing medium (line feed) in the sub-scan direction by an amount corresponding to a blank space portion of an image, for example. Then, the raster image processing generates print job data which can be transferred to the printing apparatus, by arranging the compressed printing data and the command for sub-scan direction movement.
Meanwhile, a printing apparatus which has received print job data reproduces the binary data for printing by decompressing the compressed printing data and performs the multi-pass printing by using the mask pattern for this data. Further, the printing apparatus conveys (line-feed) the printing medium in the sub-scan direction by a designated amount according to the sub-scan direction movement command.
At this time, the designated movement amount in the sub-scan direction does not always correspond to the sub-scan direction size of the mask pattern or the dot arrangement pattern or the number of printing elements arranged in the printing head. Accordingly, depending on the movement amount in the sub-scan direction, the start position of the binary data arranged after the sub-scan direction movement command is sometimes shifted from the mask pattern prepared in the printing apparatus.
FIGS. 15A to 15C are schematic diagrams for explaining a state of the shift between a dot arrangement pattern and a mask pattern caused by such sub-scan direction movement.
FIG. 15A is a diagram showing an arrangement state of the dot arrangement pattern and the mask pattern when the raster image processing instructs the sub-scan direction movement during the processing while using the dot arrangement pattern of 4×4 pixels. Each of numerals 231 to 234 indicates a pixel area in the dot arrangement pattern of 4×4 pixel area and is expressed with the binary data converted by the index processing. Numeral 235 indicates a sub-scan direction movement instructed between the dot arrangement pattern 233 and the dot arrangement pattern 234. These 231 to 235 are instructed in the raster image processing.
Meanwhile, numeral 236 indicates a mask pattern prepared by the printing apparatus. The mask pattern 236 has a unit of 4×4 pixels having the same size as that of the dot arrangement pattern to be associated with the dot arrangement pattern, and has a configuration in which this unit is continuously disposed in a width corresponding to a printing width of the printing head. Here is shown a case of using a printing head having the number of printing elements corresponding to 16 pixels, and numeral 241 indicates an area which can be printed by one main scan of the printing head. In the mask processing performed in the printing apparatus, the top of the mask pattern 236 is disposed so as to match the top of the first dot arrangement pattern 231, as shown in the drawing.
In this manner, when the instruction 235 of the sub-scan direction movement which exceeds the printing width of the printing head is included in the continuously disposed binary data sets, it is possible to match the top of the dot arrangement pattern 234 after the movement with that of the mask pattern 236 in the next main scan. Accordingly, a shift does not occur in the positional relationship between the dot arrangement pattern having a unit of 4×4 pixels and the mask pattern.
On the other hand, FIG. 15B is a diagram showing a state of the arrangement of the dot arrangement pattern and the mask pattern when the raster image processing instructs the sub-scan direction movement having an amount which does not exceed the printing width of the printing head (here, 2 pixels) in the continuously disposed dot arrangement patterns. When the instruction 235 of the sub-scan direction movement corresponding to two pixels is included in the continuously disposed binary data in this manner, the dot arrangement pattern 234 is disposed at a position shifted by two pixels from the 4×4 pixels forming the mask pattern 236. Then, when the printing is carried out in this state, it becomes impossible to control the ink application order and the like for a unit area.
Further, FIG. 15C is a diagram showing a state of the arrangement of the dot arrangement pattern and the mask pattern when a null raster is detected and the raster image processing program instructs this null raster not as the binary data but as the sub-scan direction movement. In this case, when a raster 2371 is a null, the raster is counted as a shift amount in the sub-scan direction even if the raster forms a part of the dot arrangement pattern 237, and a shift occurs from the mask pattern 236. That is, in the dot arrangement pattern 237 and the succeeding dot arrangement patterns, it becomes impossible to control the ink application order and the like for a unit area.
Each of the phenomena as explained above is caused by that information about the position of the dot arrangement pattern generated by the raster image processing is not provided accurately to the printing apparatus side.